after upgrade to 6.1.0, snapshot creation fails with "pre-save failed: qxl"
Description of problem:
When trying to create a snapshot using `virsh --connect qemu:///system snapshot-create-as <domain-name> <snapshot-name>` or virt-manager GUI, I get the following error:

```
Error: Error while writing VM state: Unknown error -1


Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 65, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/details/snapshots.py", line 237, in _do_create_snapshot
    self.vm.create_snapshot(xml)
  File "/usr/share/virt-manager/virtManager/object/domain.py", line 1124, in create_snapshot
    self._backend.snapshotCreateXML(xml, flags)
  File "/usr/lib/python3.9/site-packages/libvirt.py", line 3059, in snapshotCreateXML
    raise libvirtError('virDomainSnapshotCreateXML() failed')
libvirt.libvirtError: operation failed: Failed to take snapshot: pre-save failed: qxl
Error: Error while writing VM state: Unknown error -1
```
Additional information:
I'm using Arch Linux distro packages.
The issue appeared after upgrading qemu-headless from 6.0.0 to 6.1.0.
Downgrading back to 6.0.0 fixes the problem (snapshot are created
successfully and work as expected).

In a reply to my message to libvirt-users describing the issue [1],
Daniel P. Berrangé confirmed that the error comes from QEMU and
recommended reporting it here.

[1] https://listman.redhat.com/archives/libvirt-users/2021-September/msg00007.html